1. Technical Field of the Invention
The present disclosure generally relates to a developing unit for use in an image forming apparatus, and more particularly to an image forming apparatus including a developing unit using a two-component developer having a toner and a magnetic carrier.
2. Discussion of the Background
In general, an image forming apparatus includes a developing unit using a two-component developer (i.e., toner and magnetic carrier), for example, as shown in FIG. 1. The developing unit 5 shown in FIG. 1 includes a developing roller 5R, a delivery/recovery screw 401, and an agitation screw 11. The developing roller 5R carries a developing agent to be supplied to an image carrying member (not shown), which forms a latent image on its surface. The supply/recovery screw 401 supplies the developing agent to the developing roller 5R, and recovers the developing agent from the developing roller 5R.
The agitation screw 11 receives the developing agent from the supply/recovery screw 401 from downstream of the supply/recovery screw 401. Then the agitation screw 11 transports the developing agent in a direction opposite to a transportation direction of the supply/recovery screw 401. The agitation screw 11 also agitates the developing agent and fresh toners, which are supplied to the agitation screw 11, as required. The supply/recovery screw 401 and agitation screw 11 are arranged side-by-side in a horizontal direction as shown in FIG. 1.
As shown in FIG. 1, the supply/recovery screw 401 is provided in a supply/recovery compartment 402, and the agitation screw 11 is provided in an agitation compartment 10, where the supply/recovery compartment 402 and the agitation compartment 10 are separated by a separation wall 403. The agitation compartment 10 and the supply/recovery compartment 402 are connected to each other at both end portions of the separation wall 403, at which openings are provided in the separation wall 403 so that the developing agent can be moved through such openings. Accordingly, the developing agent can be circulated between the agitation compartment 10 and the supply/recovery compartment 402.
In the developing unit 5 shown in FIG. 1, the supply/recovery screw 401 and supply/recovery compartment 402 are used to supply the developing agent to the developing roller 5R and to recover used developing agent, where the used developing agent is an agent which is used for developing a latent image. Such used developing agent can include less amount of toner as compared to a developing agent before being used for a developing operation because some toners in the developing agent are consumed during developing of the latent image.
As shown in FIG. 1, the developing agent is supplied to the developing roller 5R and recovered from the developing roller 5R by a same screw (i.e., supply/recovery screw 401) provided in a single compartment (i.e., supply/recovery compartment 402). With such a configuration, unused developing agent and used developing agent are mixed in the supply/recovery compartment 402. Accordingly, the toner concentration in the developing agent to be supplied to the developing roller 5R may have an uneven distribution along an axial direction of the developing roller 5R. For example, the toner concentration in the developing agent may become lower from an upstream side to a downstream side of the supply/recovery screw 401.
Thus, if an image having a larger image area is developed by the developing unit 5, the toner concentration in the developing agent may significantly vary before and after a developing operation. Specifically, the toner concentration in the developing agent at the downstream side of the supply/recovery screw 401 may significantly become smaller, and thus an image quality may not be maintained at a preferable level.
Such lower toner concentration phenomenon may be prevented by increasing an amount of developing agent to be transported in the developing unit 5. However, if the amount of developing agent to be transported in the developing unit 5 is increased, the developing agent may receive a higher stress, by which a lifetime of the developing agent may become shorter.
Such a drawback may be suppressed by providing a supply screw for supplying a developing agent to a developing roller, and a recovery screw for recovering used developing agent, separately in a separate compartment. Examples of such configuration are explained below.
One such developing unit is shown in FIG. 2. A developing unit 5A shown in FIG. 2 includes a developing roller 5R, a supply screw 8, and a recovery/agitation screw 110. The supply screw 8 transports a developing agent in one direction and supplies the developing agent to the developing roller 5R. The recovery/agitation screw 110 recovers the developing agent from the developing roller 5R and transports the recovered developing agent in a direction opposite to a transport direction of the supply screw 8.
As shown in FIG. 2, the supply screw 8 is provided in a supply compartment 9, and the recovery/agitation screw 110 is provided in a recovery/agitation compartment 210, where the supply compartment 9 and recovery/agitation compartment 210 are separated by a separation wall 403. As shown in FIG. 2, the supply compartment 9 is provided over the recovery/agitation compartment 210. The supply compartment 9 and the recovery/agitation compartment 210 are connected to each other at an opening provided at both end portions of the separation wall 403.
Excessive developing agent, which is not used for developing and transported to a downstream end of the supply compartment 9, is dropped and supplied to the recovery/agitation compartment 210 from the supply compartment 9 through an opening at the downstream end of the supply compartment 9. In the recovery/agitation compartment 210, the excessive developing agent and the recovered developing agent are agitatingly transported by the recovery/agitation screw 110. At a downstream end of the recovery/agitation compartment 210, the developing agent is pushed and piled up by the recovery/agitation screw 110 so that the developing agent can be supplied to the supply compartment 9 through an opening at the downstream end of the recovery/agitation compartment 210. The recovery/agitation compartment 210 is also provided with a screw 209 as shown in FIG. 2, which is used to transport developing agent to an upstream end of the recovery/agitation compartment 210 to prevent a piling-up of too much developing agent at the downstream end of the recovery/agitation compartment 210.
In the developing unit 5A shown in FIG. 2, used developing agent is recovered in the recovery/agitation compartment 210, by which the used developing agent may not intrude in the supply compartment 9. With such configuration, the toner concentration in the developing agent in the supply compartment 9 may be maintained at a given level, and thereby the toner concentration in the developing agent to be supplied to the developing roller 5R may be maintained at a given level.
Another developing unit is shown in FIG. 3. A developing unit 5B shown in FIG. 3 includes a developing roller 5R, a supply screw 8, a recovery screw 6, and an agitation screw 11. The recovery screw 6 recovers developing agent from the developing roller 5R, and transports the recovered developing agent in a same transportation direction of the supply screw 8. Excessive developing agent, transported to a downstream end of the supply screw 8, and the recovered developing agent, transported to a downstream end of the recovery screw 6, are moved to the agitation screw 11, and then the agitation screw 11 agitatingly transports the developing agent in a direction opposite to a transport direction of the supply screw 8.
As shown in FIG. 3, the supply screw 8 is provided in a supply compartment 9, and the agitation screw 11 is provided in an agitation compartment 10, where the supply compartment 9 and agitation compartment 10 are separated by a first separation wall 404. As shown in FIG. 3, the supply compartment 9 is provided over the agitation compartment 10. The supply compartment 9 and agitation compartment 10 are connected to each other at an opening provided at both end portions of the first separation wall 404.
An excessive developing agent, which is not used for developing and transported to a downstream end of the supply compartment 9, is dropped and supplied to the agitation compartment 10 from the supply compartment 9 through an opening at a downstream end of the supply compartment 9. As shown in FIG. 3, the recovery screw 6 is provided in a recovery compartment 7, where the recovery compartment 7 and agitation compartment 10 are arranged side-by-side in a horizontal direction, and the recovery compartment 7 and agitation compartment 10 are separated by a second separation wall 405. The recovery compartment 7 and agitation compartment 10 are connected to each other at an opening provided at a downstream end of the recovery screw 6 (i.e., an end of the second separation wall 405).
Recovered developing agent is transported to the downstream end of the recovery compartment 7, and then moved to the agitation compartment 10 in a horizontal direction. In the agitation compartment 10, the agitation screw 11 agitatingly transports the developing agent. At a downstream end of the agitation compartment 10, the developing agent is pushed and piled up by the agitation screw 11 so that the developing agent can be supplied to the supply compartment 9 through an opening at the downstream end of the agitation compartment 10.
In the developing unit 5B shown in FIG. 3, used developing agent is recovered in the recovery compartment 7, by which the used developing agent may not intrude in the supply compartment 9. With such a configuration, the toner concentration in the developing agent in the supply compartment 9 may be maintained at a given level, and thereby the toner concentration in the developing agent to be supplied to the developing roller 5R may be maintained at a given level.
Another developing unit is shown in FIG. 4. A developing unit 5C shown in FIG. 4 includes a developing roller 5R, a supply screw 440, a transport screw 450, a recovery screw 6, and an agitation screw 11. The supply screw 440 transports and supplies a developing agent to the developing roller 5R. The transport screw 450, provided in a parallel position of the supply screw 440, transports the developing agent in a same transportation direction of the supply screw 440. Developing agent not used for developing operation is transported to a downstream end of the supply screw 440 and transport screw 450 as excessive developing agent.
The recovery screw 6, provided over the transport screw 450, recovers the developing agent from an upper side of the developing roller 5R, and transports the recovered developing agent in a same transportation direction of the supply screw 440 and transport screw 450. The agitation screw 11, provided next to the transport screw 450, receives the excessive developing agent and recovered developing agent transported to the downstream end of the recovery screw 6. Then, the agitation screw 11 agitatingly transports the developing agent in an opposite transportation direction of the supply screw 440 and transport screw 450.
As shown in FIG. 4, the supply screw 440, transport screw 450, and agitation screw 11 are arranged in a substantially horizontal direction. The supply screw 440 and transport screw 450 are provided in a supply compartment 9, and the agitation screw 11 is provided in an agitation compartment 10, where the supply compartment 9 and agitation compartment 10 are separated by a first separation wall 404. The supply compartment 9 and agitation compartment 10 are connected to each other at an opening provided at both ends of the first separation wall 404. The excessive developing agent, not used for developing operation, is transported to a downstream end of the supply compartment 9, and then moved in a horizontal direction through the opening at the downstream end of the supply compartment 9, and supplied into the agitation compartment 10.
The recovery screw 6 is provided in a recovery compartment 7, which is provided over the supply compartment 9. The recovery compartment 7 and supply compartment 9 are separated by a second separation wall 405. The second separation wall 405 has an opening at a downstream end of the recovery screw 6 (i.e., downstream end of the recovery compartment 7). The downstream end of the recovery compartment 7 is connected to the agitation compartment 10 via the downstream end of the supply compartment 9.
Recovered developing agent, transported to the downstream end of the recovery compartment 7, drops into the supply compartment 9 through the opening of the second separation wall 405, and then is supplied to the agitation compartment 10 with the excessive developing agent. The excessive developing agent and recovered developing agent supplied in the agitation compartment 10 are agitatingly transported by the agitation screw 11, and moved into a horizontal direction through the opening at a downstream end of the agitation compartment 10, and supplied into an upstream of the supply compartment 9.
In the developing unit 5C shown in FIG. 4, the developing agent used for developing operation is recovered in the recovery compartment 7, by which an intrusion of the used developing agent into the supply compartment 9 can be suppressed. Accordingly, compared to the developing unit 5 shown in FIG. 1, the developing unit 5C shown in FIG. 4 may reduce a variation of toner concentration in the developing agent to be supplied to the developing roller 5R.
In the developing unit 5A shown in FIG. 2, the recovery/agitation compartment 210 is used for recovering and agitation of the developing agent. With such configuration shown in FIG. 2, the developing agent recovered from the developing roller 5R may drop into the recovery/agitation compartment 210 while the recovery/agitation screw 110 is conducting an agitation of the developing agent. In such condition, developing agent, not effectively agitated by the recovery/agitation screw 110, may be supplied to supply compartment 9 from the recovery/agitation compartment 210 because some developing agent may drop from the developing roller 5R in the downstream side of the recovery/agitation compartment 210, where the developing agent dropped in the downstream side of the recovery/agitation compartment 210 may not be effectively agitated. If such developing agent is supplied to the supply compartment 9, a toner concentration in the developing agent in the supply compartment 9 may become lower or may show an uneven concentration distribution. Such phenomenon may become more serious when an image having a larger image area is produced by an image forming apparatus because the larger image area may consume more toner in the developing agent in general.
In the developing unit 5A, the supply compartment 9 is provided over the recovery/agitation compartment 210 to reduce a space volume of the developing unit 5A in a horizontal direction. The developing agent can be moved from the recovery/agitation compartment 210 to the supply compartment 9 as below: the developing agent is accumulated at the downstream of the recovery/agitation compartment 210; and then the recovery/agitation screw 110 pushes and piles up the developing agent. Accordingly, the developing agent is pushed in an upward direction to supply the developing agent from the recovery/agitation compartment 210 to the supply compartment 9, by which the developing agent may receive a larger stress, and may shorten its lifetime due to such stress.
In the developing unit 5B shown in FIG. 3, a recovery of developing agent and an agitation of developing agent are separately conducted in the recovery compartment 7 and agitation compartment 10. With such configuration, developing agent not agitated effectively may not be supplied to the supply compartment 9, by which a toner concentration in the developing agent in the supply compartment 9 may not become lower or may not show an uneven concentration distribution. However, because the supply compartment 9 is provided over the agitation compartment 10, the developing agent is pushed in an upward direction to supply the developing agent from the agitation compartment 10 to the supply compartment 9, by which the developing agent may receive a larger stress, and may shorten its lifetime due to such stress as similar to the developing unit 5A shown in FIG. 2.
In the developing unit 5C shown in FIG. 4, a recovery of developing agent and an agitation of developing agent are separately conducted in the recovery compartment 7 and agitation compartment 10. With such configuration, as similar to the developing unit 5B shown in FIG. 3, developing agent not agitated effectively may not be supplied to supply compartment 9, by which a toner concentration in the developing agent in the supply compartment 9 may not become lower or may not show an uneven concentration distribution. Furthermore, because the supply compartment 9 and agitation compartment 10 are arranged in a substantially horizontal direction, the developing agent is not supplied to an upward direction when circulating the developing agent in the developing unit 5C, by which the developing agent may not receive a larger stress, and may not shorten its lifetime.
However, in the developing unit 5C, used developing agent is recovered from an upper side the developing roller 5R in the recovery compartment 7 as shown in FIG. 4. Specifically, developing agent is recovered from a surface of the developing roller 5R, which faces upward in FIG. 4. Therefore, some used developing agent may remain on the developing roller 5R even if a magnetic force on the developing roller 5R may not effect to the developing agent. Such developing agent remained on the developing roller 5R may be transported with a rotation of the developing roller 5R, and may drop into the supply compartment 9. If the used developing agent intrudes the supply compartment 9, a toner concentration in the developing agent in the supply compartment 9 may become lower or may show an uneven concentration distribution.